The present invention pertains generally to steam desuperheaters and, more particularly, to a uniquely configured valve element for use in a nozzle assembly for a steam desuperheating device. The nozzle assembly is specifically adapted for creating a substantially uniformly distributed spray of cooling water for spraying into a flow of superheated steam in order to reduce the temperature thereof.
Many industrial facilities operate with superheated steam that has a higher temperature than its saturation temperature at a given pressure. Because superheated steam can damage turbines or other downstream components, it is necessary to control the temperature of the steam. Desuperheating refers to the process of reducing the temperature of the superheated steam to a lower temperature, permitting operation of the system as intended, ensuring system protection, and correcting for unintentional deviations from the setpoint.
A steam desuperheater can lower the temperature of superheated steam by spraying cooling water into a flow of superheated steam that is passing through a steam pipe. Once the cooling water is sprayed into the flow of superheated steam, the cooling water mixes with the superheated steam and evaporates, drawing thermal energy from the steam and lowering its temperature. If the cooling water is sprayed into the superheated steam pipe as very fine water droplets or mist, then the mixing of the cooling water with the superheated steam is more uniform through the steam flow.
On the other hand, if the cooling water is sprayed into the superheated steam pipe in a streaming pattern, then the evaporation of the cooling water is greatly diminished. In addition, a streaming spray of cooling water will pass through the superheated steam flow and impact the opposite side of the steam pipe, resulting in water buildup. This water buildup can cause erosion and thermal stresses in the steam pipe that may lead to structural failure. However, if the surface area of the cooling water spray that is exposed to the superheated steam is large, which is an intended consequence of very fine droplet size, then the effectiveness of the evaporation is greatly increased.
In addition, the mixing of the cooling water with the superheated steam can be enhanced by spraying the cooling water into the steam pipe in a uniform geometrical flow pattern such that the effects of the cooling water are uniformly distributed throughout the steam flow. Likewise, a non-uniform spray pattern of cooling water will result in an uneven and poorly controlled temperature reduction throughout the flow of the superheated steam. Furthermore, the inability of the cooling water spray to efficiently evaporate in the superheated steam flow may also result in an accumulation of cooling water within the steam pipe. The accumulation of this cooling water will eventually evaporate in a non-uniform heat exchange between the water and the superheated steam, resulting in a poorly controlled temperature reduction.
Various desuperheater devices have been developed to overcome these problems. One such prior art desuperheater device attempts to avoid these problems by spraying cooling water into the steam pipe at an angle to avoid impinging the walls of the steam pipe. However, the construction of this device is complex with many parts such that the device has a high construction cost. Another prior art desuperheater device utilizes a spray tube positioned in the center of the steam pipe with multiple nozzles and a moving plug or slide member uncovering an increasing number of nozzles. Each of the nozzles is in fluid communication with a cooling water source. Although this desuperheater device may eliminate the impaction of the cooling water spray on the steam pipe walls, such a device is necessarily complex, costly to manufacture and install and requires a high degree of maintenance after installation.
As can be seen, there exists a need in the art for a desuperheater device for spraying cooling water into a flow of superheated steam that is of simple construction with relatively few components and that requires a minimal amount of maintenance. Furthermore, there exists a need in the art for a desuperheater device capable of spraying cooling water in a fine mist with very small droplets for more effective evaporation within the flow of superheated steam. Finally, there exists a need in the art for a desuperheater device capable of spraying cooling water in a geometrically uniform flow pattern for more even mixing throughout the flow of superheated steam.